1. Field of the Invention
The present invention relates to a novel opto-electronic device capable of generating an electromagnetic wave in the THz region, i.e., an ultrahigh frequency region, as well as controlling and modulating light, and controlling and modulating an electron wave, in the fields of optoelectronic devices.
2. Description of the Related Art
As a high speed phenomenon occurring in a semiconductor, reports have been made of quantum beating, which is an oscillation caused by synthesizing non-scattering, coherent electrons or polarizations during the period from excitation to relaxation. This phenomenon occurs by the following mechanism: Using two coupled levels of semiconductor quantum well structures, the coupled levels in two excited states are simultaneously excited with an ultrashort optical pulse of a duration shorter than the relaxation time. As a result, beating occurs between the two wave functions, and charges present in the wells oscillate between the coupled quantum wells at the frequency of the beat. This oscillation acts as a very small dipole antenna, generating a THz electromagnetic wave of a frequency corresponding to the oscillation period. A THz electromagnetic wave generation device utilizing this beating phenomenon has recently been reported by AT&T Bell Laboratories.
The THz electromagnetic wave generation devices using quantum beating that have thus far been reported are shown in FIGS. 1 to 5.
The quantum well structure of FIG. 1 (K. Leo et al., Phys. Rev. Lett. 66(1991), p.201 and H. G. Roskos et al., Phys. Rev. Lett. 68(1992), p. 2216) is an asymmetric coupled quantum well structure comprising two quantum wells of different layer thicknesses (i.e., a wide well "WW" of a large layer thickness w.sub.A and a narrow well "NW" of a small layer thickness w.sub.B) coupled by a thin barrier through which electrons can tunnel. The principle of its action is illustrated in FIG. 2.
That is, in this quantum well structure, when an electric field is applied, the quantized energy level of the wide well "WW" having the well width w.sub.A and the quantized energy level of the narrow well "NW" of the well width wB coincide with each other in the conduction band. This resonant coupling results in the split of the energy level of the conduction band into two energy levels E1 and E2. In the valence band of the quantum well structure, on the other hand, the quantized energy levels of holes, E0, do not coincide between the two quantum wells, so that the holes are separated between the two wells. When, in this state, the two levels are co-excited with an ultrashort pulse having a broad frequency band including the frequencies f1 represented by the equation: f1=(E1-E0)/h and f2 represented by the equation: f2=(E2-E0)/h, the wave functions .psi.1 and .psi.2 occur respectively at the same time, as shown in FIG. 2. Since their oscillation periods are different, the function obtained by the synthesis of these two wave functions, .psi.A=.psi.1+.psi.2, is expressed, at t=0, by the following equation: EQU .psi.A(t=0)=.psi.10+.psi.20 (1)
and is expressed, at t=t, by the following equation: EQU .psi.A(t)=.psi.10.multidot.exp(-i2.pi.E1.multidot.t/h)+.psi.20.multidot.exp (-i2.pi.E2.multidot.t/h) (2)
This function oscillates at its beat frequency f=(E2-E1)/h. The resulting electron wave packet oscillates between the left and right quantum wells at the beat period. When .DELTA.E=E2-E1=10 meV, the beat frequency f is about 2.4 THz. In this manner, a THz electromagnetic wave is generated.
Another type of quantum well structure is shown in FIG. 3 (P. C. M. Planken et al., Phys. Rev. Lett., 69(1992), p.3800), which is a single quantum well structure and in which beats are generated by simultaneously exciting the two energy levels, E1 and E2, of heavy holes (hh) and light holes (lh) of the valence band.
Instead of optical excitation, injection of an electron wave is under consideration as a means for simultaneously exciting two wave functions. FIGS. 4A to 4C (J. A. Alamo et al., Appl. Phys. Lett. 56(1990), p. 78 and N. Tsukada et al., Appl. Phys. Lett. 56(1990), p. 2527) show an example the electron wave coupled device which generates quantum beats by electron injection, in which FIG. 4A is a top view of the device, and FIGS. 4B and 4C are sectional views illustrating depletion layers with and without an electric field applied, respectively. FIG. 5 is an explanatory drawing for the actions of the device. Its quantum wires are quantum wells where electrons are confined in the direction of the thickness by heterobarriers laid in the direction of the layer thickness. Another dimensional confinement is performed by applying an electric field to part of the quantum well to deplete it so that electrons are confined at the boundaries of the depletion regions. This means that in FIG. 4A, the portions other than the electron waveguide are applied the electric field, and thereby depleted, while only the non-depleted portions of the electron waveguide have electrons confined to guide their propagation. By varying the electric field to be applied to the portion shown in FIG. 4A in comparison with other portions, the confinement potential can be changed to vary the degree of the coupling between the two electron waveguides. When the electric field or voltage (V) is large, as shown in FIG. 4B, the shaded depletion layers (regions) broaden deeply to separate the two electron waveguides. On the contrary, when the electric field or voltage (V) is small, as shown in FIG. 4C, the depletion layers become shallow, so that the distribution of an electron wave in one electron waveguide spreads to the next waveguide, thereby coupling the two electron waveguides. Such control of coupling of electron wave guides by applying by an electric field is described in the paper by M. Okada et al., "Japanese Journal of Applied Physics", Vol. 27, No. 12, pp. L2424, 1988).
Using the foregoing method, as shown in FIG. 4A, two electron waveguides are coupled at the portions A and B to form a coupled electron waveguide device, and an electron wave is injected through port (electrode) C to propagate it to portion A. On this occasion, as shown in FIG. 5, the two electron waveguides are coupled at the portion A, whereby energy levels E1 and E2 having a symmetric wave function .psi.1 and an asymmetric wave function .psi.2, respectively, are formed. The wave functions .psi.1 and .psi.2 are formed at the same time with the same phase when the electron wave packets are injected or excited at t=0. The function obtained by the synthesis of these two wave functions, .psi.A=.psi.1+.psi.2, is expressed, at t=0, by the following equation: EQU .psi.A(t=0)=.psi.10+.psi.20 (3)
and is expressed, at t=t, by the following equation: EQU .psi.A(t)=.psi.10.multidot.exp(-i2E1.multidot.t/h)+.psi.20.multidot.exp(-i2 .pi.E2.multidot.t/h) (4)
as in the example of FIGS. 1 and 2. As the electron wave packet propagates in the A-A' direction in FIG. 4A, it oscillates between the waveguide A-A' and the waveguide B-B'. In this case, the period of the oscillation (beat frequency) (f) is expressed by the equation:f=(E2-E1)/h. As with the previous example, the oscillation leads to the generation of a THz electromagnetic wave.
The inventors have further formed two GaAs/AlGaAs quantum wire structures in proximity to each other, and investigated their luminous properties. As a result, the inventors have found that these quantum wire structures (quantum wells) are coupled in a quantal manner (Technical Report of IEICE, Vol. 95, No. 519, pp. 13-16, and Appl. Phys. Lett. Vol.68, No.24, p.3787, June, 1996).
With the prior art, however, the period of oscillation is determined by the difference in energy between two levels, and the oscillation which will be obtained is limited for polarizations formed from electrons and holes. Thus, it has been difficult to obtain polarizations having an desired oscillation and an arbitrary time course, and it also has been difficult to manipulate the oscillation and polarization. For THz electromagnetic wave radiation accompanying the polarization oscillation, there also has a difficulty in obtaining an electromagnetic wave having an arbitrary frequency and an arbitrary waveform, and an electromagnetic wave having a variable frequency. For an excited electron wave, formation of an arbitrary electron wave has been difficult.